1. Field of the Invention
The present invention relates to a vacuum pump system for pumping gas from an enclosure and to a method of controlling the system.
2. Background of the Related Art
A vacuum pump system may be used for pumping gas from an enclosure, such as a load lock chamber or transfer chamber of a semiconductor processing assembly. The system may comprise a vacuum pump unit having a pump inlet connectable with an outlet of such an enclosure and a pump outlet for exhausting gas, usually to atmosphere.
Processing of semiconductor wafers, for instance, takes place at low pressures close to vacuum which for brevity hereinafter will be referred to as vacuum. Transferral of wafers at atmospheric pressure to processing chambers at vacuum is achieved by the use of an intermediate chamber, or load lock chamber, which is adjustable between atmosphere and vacuum. Unprocessed semiconductor wafers are deposited in a load lock chamber and processed wafers are removed therefrom when the chamber is at atmospheric pressure. Wafers can be transferred between the load lock chamber and the processing chambers when the load lock chamber has been evacuated to vacuum.
It is desirable to be able to reduce the load lock chamber from atmosphere to vacuum quickly to increase efficiency of the semiconductor processing assembly. When the load lock chamber has been evacuated to vacuum, there is little or no mass flow rate through the vacuum pump unit—the unit is maintaining inlet and outlet pressure, or is holding back outlet pressure downstream thereof. When the vacuum pump unit is operating under this condition, it is said to be operating at ‘ultimate’.
It is desirable to reduce the power requirement of a vacuum pump unit operating at ultimate. The work done by the vacuum pump unit is proportional to the change in inlet pressure to outlet pressure. Hence, if the inlet pressure is to be maintained at vacuum, the outlet pressure can be reduced to decrease the power requirement of the unit. Further, a vacuum pump system consumes less power if there is less gas in the system.
In a known arrangement, an inlet of a secondary pump unit is connectable to the pump outlet of the vacuum pump unit when operating at ultimate for reducing pressure at the pump outlet, which in turn, reduces the power requirement of the vacuum pump unit. This arrangement is undesirable from a number of standpoints. There is an additional power requirement of the secondary pump and also the necessity of maintenance thereof. Furthermore, there is the problem of accommodating the additional foot print of the secondary pump.